One Way Slab

In reinforced concrete design, a slab is said to be one way if the length of the longer side divided by the length of the shorter side is equal to or greater than 2. 

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With reference to the diagram of the slab shown below, If the length of the longer side (Ly) = 5m, and the shorter side (Lx) is 2.5m then 

Ly/Lx = 5m/2.5m = 2

On structural plans, the symbol used to identify a one way slab panel is shown in the previous diagram (symbol that the arrow points at).

In a one way slab design, the main reinforcement steel spans along the shorter side of the slab, which is also the span of the slab

In a one way slab design, only the two long supporting beams along the the long side of the slab  is assumed to carry the slab load equally between them. The beams along the shorter side do not carry any slab load.

For instance with reference to this diagram of a one way slab, beam 1 carries half of the slab load and beam 2 takes the remaining half, none of the slab load is carried by beams 3&4. This makes beam 1&2 the critical beams. Beams 3&4 will not be carrying any slab load, but may carry wall load and part of roof load. 

Two Way Slab

If the length of the longer side divided by that of the shorter side is less than 2, the slab should be designed as a two way.

With reference to the diagram shown, the length of the longer side, Ly, =6, and that of the shorter side Lx = 4.75. Therefore, Ly / Lx = 6 / 4.75 = 1.26. 1.26 is less than 2 hence the slab should be designed as 2 way.

The symbol of a two way slab is as shown above (the symbol that the arrow points at);

In a 2 way slab design, the main steel reinforcement spans both directions.

All 4 supporting beams carry some part of the slab load but the beams along the longer side carry more of the slab load than the beams along the shorter side.

The loading on beams for a 2 way slab is as shown. Note that the beams 1& 2 carry more slab load than beams 3&4